1. Field of the Invention
The present invention relates to a silver halide emulsion, and a silver halide photosensitive material. More specifically, the present invention relates to a silver halide emulsion which is suitable for quick processing and provides a high sensitivity and a high gradation even in digital exposure such as laser scanning exposure, and a silver halide photosensitive material using the silver halide emulsion.
2. Description of the Related Art
In recent years, digitization has been remarkably widespread in the field of a color print using a color photographic printing paper. For example, a digital exposure system using laser scanning exposure has been rapidly spread in comparison with an ordinary analog exposure system in which printing is directly conducted from a processed color negative film with a color printer. Such a digital exposure system is characterized in that a high image quality is obtained by image processing, and greatly contributes to improving qualities of a color print using a color photographic printing paper. Further, according to the rapid spread of digital cameras, it is also an important factor that a color print with a high image quality is easily obtained from these electronic recording mediums. It is believed that they will rapidly spread further.
With respect to a color print system, techniques such as an ink jet system, a sublimation system, a color xerography and the like have made progress, and these are being recognized as a color print system with a good photographic image quality. Of these, a digital exposure system using a color photographic printing paper has characteristics such as a high image quality, a high productivity and a high fastness of an image. It has been required to provide higher-quality photographs more easily at lower cost by making use of these characteristics. Further, unlike a silver halide sensitive material for photography of a color negative, a blank of a color printing paper to be directly observed tends to show clearly. Accordingly, for competition with other printing materials, it is important to decrease a blank density by reducing a residual color of a dye or a sensitization coloring matter to decrease a fog density.
As a silver halide emulsion used in a color photographic printing paper, a silver halide emulsion having a high content of silver chloride has been used mainly to meet a requirement of having a quick processability for increasing a productivity. Especially, the use of a silver halide emulsion having a high content of silver chloride and a small particle size further improves the quick processability and provides a high color formation efficiency, making it possible to provide a less costly color photographic printing paper. Accordingly, an increase in the performance of a silver halide emulsion having a high content of silver chloride and a small particle size is important for providing high-quality photographs more easily at lower cost. However, such a silver halide emulsion having a high content of silver chloride and a small particle size has been problematic in that it has a low sensitivity and is liable to induce a low sensitivity and a soft gradation through high-intensity exposure such as laser scanning exposure. Further, for quick return of a color print, it is important to conduct development for a short period of time after exposure in a mini-laboratory system. Nevertheless, the silver halide emulsion having a high content of silver chloride and a small particle size has been problematic in that a latent image stability is poor for several seconds to several dozen seconds after exposure.
It has been known that iridium can be doped for improving high intensity failure of a silver chloride emulsion. However, it has been also known that a silver chloride emulsion doped with iridium causes sensitization of a latent image for a short period of time after exposure. For example, JP-B No. 7-34103 discloses that a localized phase having a high content of silver bromide is provided and doped with iridium to solve the problem of latent image sensitization. A silver halide emulsion formed by this method provides a high sensitivity and a high gradation even by relatively high-intensity exposure for approximately 1/100 second without the problem of the latent image sensitization. However, it has been clarified that when an attempt is made to maintain a high sensitivity by ultrahigh-intensity exposure for 1 microsecond required in a digital exposure system by laser scanning exposure, a high gradation is hardly obtained. Moreover, U.S. Pat. No. 5,691,119 discloses a method of preparing an emulsion having a localized phase with a high content of silver bromide to provide a high gradation. However, it is problematic in that the effect is insufficient and a performance is not stabilized by repeated preparation.
U.S. Pat. Nos. 5,783,373 and 5,783,378 disclose that high intensity failure is decreased by using at least three types of dopants to provide a high gradation. However, a high gradation is obtained because of the use of a dopant having a low sensitivity and a high gradation which is in principle contrary to a high sensitivity.
U.S. Pat. Nos. 5,726,005 and 5,736,310 disclose that an emulsion having a high sensitivity with high intensity failure reduced is obtained with an emulsion containing I in which a concentration maximum is present on a sub-surface of a high silver chloride emulsion. Examples of EP 0,928,988A demonstrate that an emulsion excellent in reciprocity law failure, temperature dependence of exposure and pressure property is obtained by incorporating a specific compound in particles having a side length of 0.218 μm, namely, a sphere-equivalent diameter of approximately 0.27 μm in which an I band is formed in 93% of particle formation. In the silver halide emulsion having a high content of silver chloride and a small particle size as described in these documents, higher intensity exposure leads to a higher sensitivity. However, it has been found that ultrahigh-intensity exposure such as laser scanning exposure provides quite a soft gradation which is not suitable for digital exposure with a limited dynamic range of an amount of light and a latent image stability is poor for several seconds to several tens of seconds after exposure. Further, U.S. Pat. Nos. 5,728,516, 5,547,827 and 5,605,789 and JP-A No. 8-234354 disclose a method of decreasing a fog density of an emulsion containing I in which a concentration maximum is present on a sub-surface of a high silver chloride emulsion. However, satisfactory effects have not been provided when using it as a printing material.
JP-A Nos. 58-95736, 58-108533, 60-222844, 60-222845, 62-253143, 62-253144, 62-253166, 62-254139, 63-46440, 63-46441 and 63-89840 and U.S. Pat. Nos. 4,820,624, 4,865,962, 5,399,475 and 5,284,743 disclose that a high sensitivity is provided by localizing a phase having a high content of silver bromide in an emulsion having a high content of silver chloride in various modes. However, these documents do not disclose that a peculiar function of a high gradation is provided in ultrahigh-intensity exposure such as laser scanning exposure with a silver halide emulsion containing a silver bromide-containing phase and/or a silver iodide-containing phase and having a high content of silver chloride and a small particle size. Further, in these silver bromide-containing phases, the distribution inside the particle is not optimized. Thus, the effects have been unsatisfactory in ultrahigh-intensity exposure such as laser scanning exposure.
U.S. Pat. No. 5,049,485 discloses that a high sensitivity and a high gradation are provided by chemical sensitization with an Au (I) compound coordinated with a mesoion. U.S. Pat. No. 5,945,270 discloses that a high sensitivity and a high gradation are provided by chemical sensitization with an Au(I) compound coordinated with mercapto containing a water-soluble group. These compounds are known to be relatively stable Au (I) compounds. However, these documents do not disclose that a peculiar function of a high gradation is provided in high-intensity exposure with an emulsion containing a silver bromide-containing phase and/or a silver iodide-containing phase.